Free SSL Certificate Checker
Inspect the TLS certificate presented on port 443 to confirm certificate identity, issuer, validity windows, and chain details before users hit trust errors.
Common use cases
- Confirm a certificate was installed correctly after renewal or migration.
- Review expiry dates and issuer details before planned maintenance.
- Validate that a hostname is covered by the certificate subject or SANs.
The SSL checker connects to the public host on port 443 and inspects the certificate presented during the TLS handshake, including identity, issuer, validity dates, alternative names, and chain information available to the client.
What a healthy certificate proves
The certificate should be currently valid, cover the exact hostname, chain to a trusted issuer, and use contemporary cryptography. It proves server identity for the connection; it does not prove that the application itself is secure.
- Check both the apex and www hostname when users can reach both.
- Allow enough renewal margin for DNS, CDN, and deployment failures.
- Verify intermediate certificates after changing a load balancer, CDN, or hosting provider.
Investigate failures at the edge
Different edge locations can present different certificates. SNI behavior, cached CDN configuration, IPv4 and IPv6 routing, or an incomplete intermediate chain may explain why one client succeeds while another fails.
- Test the hostname rather than only its resolved IP so SNI selects the intended certificate.
- Pair this result with TLS protocol and redirect checks.
- Use browser and server logs when a handshake fails before a certificate is returned.
Standards and further reading
Diagnostic output is evidence, not a guarantee of security, deliverability, availability, or search ranking. Read the NerdTools methodology and limitations before making consequential changes.
Frequently asked questions
What port does the SSL checker use?
The SSL checker connects to the public host on port 443 and inspects the presented certificate.
Can this detect an expired certificate?
Yes. The result includes certificate validity dates and related warnings when the certificate is not currently valid.